1. Field of the Invention
The present invention relates to an ultrasonic flowmeter for A/D-converting an ultrasonic signal, calculating a discrete correlation factor and determining a time difference equivalent to a flow velocity.
2. Description of the Prior Art
The method of determining a flow velocity, i.e., a time difference, using a prior art ultrasonic flowmeter, according to a correlation factor between ultrasonic signals passing through a fluid has the advantage of being resistant to disturbance, such an noise. In order to determine the correlation factor between ultrasonic signals, the signals must be A/D-converted and then correlated. The correlation factor, therefore, results in a discrete value. Thus, in order to evaluate a precise time difference equivalent to a flow velocity from the resulting discrete values, the discrete data must be interpolated as shown in FIG. 1. Then, the time at which the correlation factor is at a maximum must be calculated from the interpolated discrete data.
In order to evaluate the maximum value of a correlation factor, it is common to use higher-order curvilinear interpolation. This approach requires a large amount of data, however. To be able to acquire a large amount of data, it is necessary to use extremely fast, high-resolution A/D converters and make complex higher-order curvilinear interpolation calculations. Conversely, curvilinear interpolation using a small amount of data will result in a larger calculation error. Such high-speed high-resolution A/D converters are extremely costly. In addition, higher-order curvilinear interpolation calculations tend to become complex.
The present invention has been made in the light of the above-noted problems. An object of the invention is, therefore, to provide an ultrasonic flowmeter that implements the method of calculating a time difference with improved accuracy and at higher speeds using relatively simple signal processing means and thereby determining the maximum value of a correlation factor.
In order to attain the above-noted object, the ultrasonic flowmeter in a first aspect of the present invention recited in claim 1 is defined as one for determining a flow rate according to ultrasonic signals passing through a fluid, comprising;
calculating means for A/D-converting and thereby discretizing an upstream receive signal and a downstream receive signal;
correlation processing means for determining a cross-correlation between the upstream receive signal and downstream receive signal;
calculating means for Hilbert-transforming the result of calculation made by the correlation processing means;
phase calculating means for determining a phase relationship from the results of calculation provided by the correlation processing means and Hilbert transform means; and
first calculating means for calculating a time difference according to the result of phase relationship calculation made by the phase calculating means.
Therefore, according to the first aspect of the present invention recited in claim 1, upstream and downstream receive signals are discretized without having to make complex higher-order curvilinear interpolation calculations; a correlation factor is determined using correlation processing means; the result of Hilbert calculation provided by Hilbert transform means in obtained; and the maximum value of the correlation factor is determined by making a phase calculation from the correlation factor and the result of the Hilbert calculation.
In a second aspect of the present invention as recited in claim 2, the ultrasonic flowmeter of the present invention is defined an one for determining a flow rate according to ultrasonic signals passing through a fluid, comprising:
calculating means for A/D-converting and thereby discretizing an upstream receive signal and a downstream receive signal;
correlation processing means for determining a cross-correlation between the upstream receive signal and the downstream receive signal;
calculating means for Hilbert-transforming the result of calculation made by the correlation processing means;
phase calculating means for determining a phase relationship from the result of calculation made by the correlation processing means and the result of calculation provided by the Hilbert transform means;
first calculating means for determining a time difference from the result of phase relationship calculation made by the phase calculating means;
calculating means for Hilbert-transforming the upstream receive signal and the downstream receive signal;
calculating means for determining an envelope from the upstream receive signal, the downstream receive signal, and the result of calculation provided by the Hilbert transform means;
second calculating means for determining a time difference from the envelope; and
a function for determining a flow velocity dependent time difference from the time difference determined by the second calculating means and the time difference determined by the first calculating means.
Therefore, according to the second aspect of the present invention recited in claim 2, upstream and downstream receive signals are Hilbert-transformed; an envelope is determined from the result of Hilbert transform based calculation of the upstream and downstream receive signals; an approximate time difference is calculated from the envelope; an accurate time difference, which is the maximum value of the approximate time difference, in then determined from the result of phase calculation; thus making it possible to identify a true time difference when the correlation factor proves to be multiple-peaked.
In a third aspect of the present invention as recited in claim 3, the ultrasonic flowmeter of the present invention is defined as one for determining a flow rate according to ultrasonic signals passing through a fluid, comprising:
calculating means for A/D-converting and thereby discretizing an upstream receive signal and a downstream receive signal;
correlation processing means for determining a cross-correlation between the upstream receive signal and the downstream receive signal;
calculating means for Hilbert-transforming the result of calculation made by the correlation processing means;
phase calculating means for determining a phase relationship from the results of calculation provided by the correlation processing means and Hilbert transform means;
first calculating means for determining a time difference according to the result of phase relationship calculation made by the phase calculating means;
third calculating means for determining an envelope from the geometric mean of the results of calculation provided by the correlation processing means and Hilbert transform means, and then calculating a time difference from the maximum value of the envelope; and
a function for determining a flow velocity dependent time difference from the time difference determined by the third calculating means and the time difference determined by the first calculating means.
Therefore, according to the third aspect of the present Invention recited in claim 3, an envelope is determined from the geometric mean of a correlation factor obtained by the correlation processing means from the correlation between upstream and downstream receive signals and the result of calculation provided by Hilbert transform means; an approximate time difference is calculated from the maximum value of the envelope; an accurate time difference, which is the maximum value of the approximate time difference, is then determined from the result of phase calculation; thus making it possible to identify the true time difference when the correlation factor proves to be multiple-peaked.
In a fourth aspect of the present invention as recited in claim 4 the ultrasonic flowmeter of the present invention is defined as one for determining a flow rate according to ultrasonic signals passing through a fluid, comprising:
calculating means for A/D-converting and thereby discretizing an upstream receive signal and a downstream receive signal;
correlation processing means for determining a cross-correlation between the upstream receive signal and the downstream receive signal;
calculating means for Hilbert-transforming the result of calculation made by the correlation processing means;
phase calculating means for determining a phase relationship from the results of calculation provided by the correlation processing means and Hilbert transform means;
first calculating means for determining a time difference from the result of phase relationship calculation made by the phase calculating means;
fourth calculating means for determining the auto-correlations between upstream transmit and receive signals and between downstream transmit and receive signals respectively and thereby calculating a time difference; and
a function for determining a flow velocity dependent time difference according to the time difference determined by the fourth calculating means and the time difference determined by the first calculating means.
Therefore, according to the fourth aspect of the present invention recited in claim 4, the auto-correlations between upstream transmit and receive signals and between downstream transmit and receive signals are determined respectively; an approximate time difference is calculated from the auto-correlations; and the accurate time difference, which is the maximum value of the approximate time difference, is then determined from the result of phase calculation; thus making it possible to identify the true time difference when the correlation factor proves to be multi-peaked.